Numerous derivatives of organic phosphinic acids are known to exist and to have considerable commercial value as well as a great variety of useful applications. For example, organic phosphinates as well as their acids are effective wetting agents and detergents; plasticizers for many plastics and resins; bonding agents for asphalt and similar compositions; color stabilizers and oxidation inhibitors for greases and lubricants (U.S. Pat. No. 3,001,938); corrosion inhibitors; flame proofing agents; flotation auxiliaries; metal extractants; setting retarders for gypsum; and textile auxiliaries such as filament stabilizers (U.S. Pat. No. 3,374,288).
Highly purified, highly branched dialkylphosphinic acids have been especially recognized as being very important and much desired precursors, intermediate products, and end products in numerous specialized fields. For example, branched dialkylphosphinic acids act as complex-forming agents; pharmaceutically active materials, especially those suitable for the treatment of inflammations, and degenerative diseases of the joints, such as rheumatoid arthritis (U.S. Pat. No. 4,524,211); general agricultural and household chemicals including plant growth regulators, insecticides, and herbicides; and antistatic agents. In many, if not all of these applications, the presence of monoalkylphosphinic acid by-product can be detrimental due to the reactivity of the phosphorus-hydrogen moiety and the thermal instability of such compounds.
As a result of the above listed numerous possibilities of practical application, a demand has been created for a simple industrial synthesis for the production of these dialkylphosphinic acids in a highly purified state. Because of the aforedescribed great commercial value, many methods of preparing organic phosphinic acids and their phosphinates have been advanced. Although the methods vary widely in their individual steps, a great many employ the reactions of phosphorous-halogen compounds to attain carbon-to-phosphorous bonds. While it has long been known to be possible to form such bonds by reacting alkyl halides with phosphine, or by the use of Grignard reagents, such methods are not practical in commercial scale operations.
Stiles et al. (U.S. Pat. No. 2,724,718) discloses a process for the production of phosphinates employing the reaction between a compound containing olefinic double bonds and, preferably, a class of compounds consisting of compounds of the formula (I):
wherein Z represents a monovalent hydrocarbon radical free of aliphatic multiple bonds, or a monovalent inorganic cation, and Y represents a hydrogen atom, a monovalent hydrocarbon radical free of aliphatic multiple bonds, or the group-OZ in which Z is defined as above. Among the phosphorous classes and compounds that Stiles et al. suggest as reactants are the salts of hypophosphorous acid, hydrocarbyl esters of hypophosphorous acid, hydrocarbyl esters of organic phosphinic acids and mono- and di-hydrocarbyl esters of phosphorous acid. A particularly preferred subclass comprises the alkali metal salts of hypophosphorous acid such as sodium hypophosphite which Stiles et al. found to be able to be directly added to olefins containing up to 14 carbon atoms “to produce in a single, operational step a water soluble detergent in substantially quantitative yields.”
Stiles et al. also noted that 1-olefins exhibit a somewhat higher rate of reaction in these processes than do other olefins. The Stiles et al. addition reaction is initiated by the presence of free radicals in intimate contact with the reactants. Neither the reaction temperature nor the reaction pressure is taught to be critical by Stiles et al.
Stiles et al. teach that where a mole to mole addition is desired, it is generally preferable to employ the reactants in about equimolar proportions or with the phosphorous compound in excess; and, where it is desirable to cause more than one mole of the olefinic compound to be incorporated in the product, for example to produce a di-alkylphosphinic acid, it is preferable to employ about a 2 to 3 to 1 molar excess of the olefinic compound.
A. J. Robertson (U.S. Pat. No. 4,374,780) discloses the production of a highly branched, dialkyl phosphinic acid namely di-2,4,4′-trimethylpentyl phosphinic acid by the free radical addition of two moles of an alkene, specifically 2,4,4′-trimethylpentene-1, to phosphine gas followed by an oxidation of the phosphine reaction product to the phosphinic acid using two moles of hydrogen peroxide. It is disclosed, however, that high phosphine pressures, i.e., up to about 1000 psig may be necessary to achieve high phosphine to olefin ratios and thus reduce unwanted tri-2,4,4′-trimethylpentylphosphine by-product; for any such by-product formed is a total yield loss. Also, the exothermic oxidation stage is said to be temperature critical for if the temperature exceeds about 120° C., an alkyl group is removed and additional monoalkylphosphinic acid is formed; temperatures below about 50° C., result in excessive reaction times. A straight forward distillation was said to be able to achieve good dialkylphosphinic acid yields.
Of course, monoalkyl- and dialkylphosphinic acids could also be formed by hydrolytic cleavage of the respective alkyl esters, whose phosphorous-carbon bonds had been formed in the first place by other means, at temperatures of from about 160° C. to 300° C. using at least a quantity of water which is required by stoichiometry for the hydrolysis. The alkanol formed as one of the hydrolysis products is usually removed from the reaction mixture by distillation. (U.S. Pat. No. 4,069,247).
Alkyl phosphinic acids have also been used to extract rare earth elements (U.S. Pat. No. 5,639,433). In the general procedure employed for the separation of rare earth elements from solutions thereof, especially acidic solutions, the feed solution generally results from the treatment of ores containing rare earth elements such as monazite, bastnaesite, xenotime, bauxite, and similar crude ores. The extract containing the extracted rare earth element(s) is usually sent to a scrubber wherein it is scrubbed with dilute acid and then sent to a stopper where it is stripped with more concentrated acid to separate the rare earth elements. Hydrochloric acid is the preferred acid of the prior art to scrub and strip the extract. Bis-(2,4,4-trimethylpentyl)phosphinic acid is said to be a preferred extractant; especially for the separation of cobalt from nickel.
Further, with respect to end uses of the dialkylphosphinic acids and their esters, U.S. Pat. No. 6,165,427 discloses the use of a composition comprising sodium di-(n-octyl)phosphinate and sodium di-(n-dodecyl)phosphinate to precipitate and recover soluble heavy metals such as lead, cadmium, zinc species and mixtures thereof from wastewater streams. It is taught that advantageously, the organophosphorus salts may be regenerated from the precipitate by treating the precipitate with concentrated aqueous hydroxide to dissolve it and then contacting the resulting solution with diethyl ether in, for example, a separation funnel. After agitation and subsequent phase disengagement, two phases are present. One phase is an aqueous phase containing the metal with a concentration higher than that of the feed. The other phase is the ether solution of the precipitating agent. The ether is evaporated and the sodium di-(alkyl)phosphinate is regenerated.
Purifications of the alkyl phosphinic acids and their esters are often accomplished via additions of an organic material such as diisopropyl ether or petroleum ether (U.S. Pat. No. 4,434,108); followed by repeated evaporations, crystallizations, and filtrations (U.S. Pat. No. 4,524,211).
The major problem inherent in the aforedescribed processes of the prior art, is that it is extremely difficult to separate the di-alkylphosphinic acids from co-formed mono-alkyl reaction products since they have very similar aqueous solubilities. This art-recognized problem of producing high purity dialkyl phosphinic acids by a practical reaction process which is applicable to the production of compounds having a variety of structures, especially highly branched dialkyl structures, has heretofore remained unsolved.
Accordingly, it is an object of this invention to provide a practical and efficient process for addressing this technical problem by providing conditions whereby, in a straightforward alpha olefin-hypophosphorous acid or a salt thereof free radical reaction, any monoalkylphosphinic acid and other water soluble impurities present are removed from the di-alkylphosphinic acid product by a simple neutralization/phase separation without the need for a third component organic solvent addition.
Other objects will be evident from the ensuing description and appended claims.